1. Field of the Invention
The present invention relates to a multi-parallel magnetic-field cancellation type transformer for converting a voltage and an electric power conversion circuit including the transformer.
2. Description of the Related Art
Heretofore, there is known an electric power conversion circuit, a so-called DC-DC converter, which converts electric power by stepping up/down an input voltage (for example, JP2006-149054A discloses the DC-DC converter).
The conventional DC-DC converter disclosed in JP2006-149054A includes a transformer in which a primary coil and a secondary coil are connected to each other, and an inductor applied for varying a step-up/down rate and disposed between the transformer and an input-output terminal to which a voltage is applied. The conventional DC-DC converter can successively step up/down the voltage and reduce the size of the inductor, which leads to the downsized DC-DC converter.
However, according to the conventional DC-DC converter, if the step-up/down rate is equal to or greater than two times, the simultaneous ON state of a plurality of switching elements connected to the coils of the transformer causes a ripple current to increase. The ripple current is a pulsation component to be superimposed on a direct current flowing through the inductor applied for varying the step-up/down rate. Accordingly, the conventional DC-DC converter cannot smooth out the pulsation (variation) of the ripple current without increasing the body dimension (size) of a capacitor or an inductor. Therefore, it is difficult to reduce the size of such passive elements as capacitors and inductors and reduce the size of the DC-DC converter disclosed in JP2006-149054.
Further, according to the conventional DC-DC converter, when the step-up/down rate is equal to or greater than two times, an increase of the ripple current leads to the deterioration of electric power conversion efficiency.